This invention relates generally to electroluminescent lamp drivers and, more particularly, to a DC to AC switching circuit for driving a load exhibiting capacitive characteristics.
An electroluminescent (EL) lamp is an increasingly common light emitting device utilized for providing display backlighting in many types of battery powered devices such as watches, PDAs, and cellular telephones, for example. The electrical loading characteristics of an EL lamp are substantially capacitive. To excite an EL lamp into luminescence, a relatively high voltage AC signal must be applied to the terminals thereof. Typically, the AC signal will exhibit an oscillation frequency within the range of 200-400 Hz with a peak-to-peak amplitude of 100-200 volts. To generate a high voltage AC signal from the relatively low DC voltage typically supplied by a battery, it is common to step the low battery voltage up to a relatvely high DC voltage utilizing a common boost converter. The DC voltage provided at the output of the boost converter is then converted to an AC voltage using any of a number of well known switching techniques, The H-bridge represents one such switching circuit.
Several prior art methods are known for converting a DC voltage to the AC voltage required for driving an electroluminescent lamp. Exemplary of the prior art is that described in U.S. Pat. Nos. 4,527,096 to Kindlmann and 5,463,283 to Sanderson. Kindlmann teaches a circuit for efficiently driving an EL lamp utilizing relatively few components. However, in the Kindlmann circuit, relatively large current pulses are delivered directly from an inductive element into the capacitve EL lamp. These large current spikes produce a series of small voltage steps across the EL lamp. From a reliability standpoint, these large current spikes could shorten the life of the EL lamp. The Sanderson patent discloses an alternate approach in which a common boost converter generates a relatively high voltage DC supply which is utilized to sequentially charge and discharge the EL lamp by means of a constant current through an H-bridge switching circuit. Therefore, in contrast to delivering energy through a series of large current spikes, as taught by Kindlmann, Sanderson's approach delivers energy to the EL lamp by means of a constant current, thereby extending the life of the EL lamp. Nevertheless, the Sanderson circuit is disadvantageous in that his use of current from the high voltage supply to remove charge from the capactive EL lamp will result in significantly higher power consumption when compared to the Kindlmann circuit.
It would therefore be advantageous to provide a switching circuit in accordance with the present invention for delivering energy from a high-voltage DC supply to an EL lamp utilizing a constant current flow and an improved method for discharging the capacitive EL lamp to thereby achieve significant reductions in the average current consumption from the high voltage supply.